1. Field of the Invention
The present invention relates to a vacuum covering method in which a sheet is drawn by vacuum or suction down onto a profiled surface of a base member to effect bonding therebetween to cover the profiled surface with the sheet. It also relates to a sheet particularly suitable for use in the vacuum covering method.
2. Description of the Related
FIG. 29 shows in cross section a portion of a vehicle instrument panel which exemplifies an article produced in accordance with a conventional vacuum covering method. The article (instrument panel) has a sheet bonded by adhesion to a profiled surface of a base member through the vacuum process.
Conventionally, the instrument panel is produced as follows.
At first, a base member 101 injection-molded from synthetic resin and a sheet 102 of double layered structure having a facing layer 103 and a foam layer 104 of polypropylene are provided. Then, the base member 101 is set on the lower mold member (not shown) of a vacuum mold assembly. Subsequently, a primer 105a is coated on a rear surface (foam-layer side) of the sheet 102, and an adhesive 105b is coated on a front surface of the base member 101. After heating the base member 101 and the sheet 102 at desired temperatures, the sheet 102 is placed over the base member 101. In this instance, the sheet is stretched in one direction. Then, an upper mold member is lowered toward the lower mold member to close the vacuum mold assembly with the sheet 102 and base member 101 held therein. While the vacuum mold assembly is kept closed, air is evacuated from vacuum holes 101a in the vacuum mold assembly so that the sheet is drawn by vacuum or suction down onto the base member to thereby effect adhesive bonding between the sheet 102 and the base member 101.
A typical material eligible for the base member 101 is acrylonitrile-butadiene-styrene (hereinafter abbreviated as "ABS") resin. The facing layer 103 of the sheet 102 is made from polyvinyl chloride (hereinafter abbreviated as "PVC"), and the foam layer 104 of the sheet 102 is made from polypropylene (hereinafter abbreviated as "PP").
In general, the base member 101 of ABS resin is produced by injection-molding. The injection-molded base member 101 undergoes thermal shrinkage as it becomes cool. The PP is advantageous over the ABS resin from a recycling point of view. However, the PP is a crystalline polymer and hence has a large rate of shrinkage. Accordingly, in the case of a base member 101 made from PP, it occurs likely that due to thermal shrinkage concentrated on an area surrounding a vacuum hole 101a, an annular shrink mark (also called "sink mark") is formed around the vacuum hole 101. As a result of generation of the shrink mark, an edge portion 101b of the vacuum hole 101a is forced to swell or project outwardly from a surface of the base member 101. Thus, a sheet 102 which is bonded to the surface of the base member 101 including the projecting edge portion 101b necessarily has a projection 102a on its front surface at a position corresponding to the position of the projecting edge portion 101b. The projection 102a thus formed makes the sheet 102 unsightly in the appearance and lowers the product value of the instrument panel.